In the field of automotive engineering, there is an ongoing quest to reduce total vehicle weight or to add improved equipment without increasing total vehicle weight. This can only be achieved by reducing the weight of certain vehicle components. In this connection, there is particular impetus to reduce the weight of the vehicle body significantly in comparison to prior designs. At the same time, however, there are increased demands relating to safety, in particular safety to persons traveling in the vehicle, and relating to the behavior of the vehicle in an accident. While the number of parts and in particular also the thickness of parts are reduced in order to reduce the gross weight of the body, the reduced-weight body shell is expected to have an increased strength and rigidity with a definite deformation behavior in the event of an accident.
The raw material most frequently used in body manufacture is steel. No other material is able to provide so many sectors with inexpensive components that boast such a wide variety of material properties.
As a result of the changed requirements, at high strengths, high expansion factors are also assured and along with them, an improved cold formability. In addition, the range of achievable strengths for steels has been increased.
One example of this, particularly for automotive bodies, is components comprised of sheet steel with a strength—depending on the alloy composition—in a range from 1000 to 2000 MPa. In order to achieve such high strengths in the component, it is known to cut corresponding blanks from sheets, to heat the blanks to a temperature higher than the austenitizing temperature, and then to form the component in a press; during the forming process, a rapid cooling is simultaneously executed in order to harden the material.
During the annealing that is carried out in order to austenitize the plates, a scale layer forms on the surface. This is descaled after the forming and cooling. Usually sandblasting processes are used for this. Before or after this descaling, the final trimming and introduction of holes are carried out. If the final trimming and introduction of holes are carried out before the sandblasting, then this can have a disadvantageous effect on the cut edges and hole edges. Independent of the sequence of processing steps after the hardening, descaling by means of sandblasting and comparable methods has the disadvantage that this frequently distorts the component. After the above-mentioned processing steps, a so-called component coating with a corrosion protection layer is carried out. For example, a cathodic corrosion protection layer is applied.
In this context, it is disadvantageous that the remachining of the hardened component is extraordinarily expensive and involves a very high degree of wear due to the hardening of the component. It is also disadvantageous that the component coating usually does not produce a particularly pronounced corrosion protection. In addition, the layer thicknesses are not uniform, but instead fluctuate over the surface of the component.
In a modification of this method, it is also known to cold form a component out of a sheet metal blank, to subsequently heat it to the austenitizing temperature, and then to rapidly cool it in a calibration tool. The component, which experiences distortion due to the heating, is calibrated by the calibration tool in its formed regions. Then, the above-described remachining is carried out. This method enables the production of more complex geometries than the method described previously because simultaneous forming and hardening is essentially only able to produce linear forms, but such shaping procedures are unable to produce complex forms.
GB 1 490 535 has disclosed a method for manufacturing a hardened steel component in which a plate of hardenable steel is heated to the hardening temperature and then placed into a forming device in which the plate is formed into the desired final form and during the forming, is rapidly cooled at the same time so that a martensitic or bainitic structure is obtained while the sheet remains in the forming device. For example, a boron-alloyed carbon steel or carbon manganese steel is used as the base material. According to the patent application mentioned above, the forming is preferably a pressing, but can also be used with other methods. The forming and cooling should preferably be designed and rapidly executed so as to obtain a fine-grained martensitic or bainitic structure.
EP 1 253 208 A1 has disclosed a method for manufacturing a hardened plate profile from a plate that is hot formed and hardened in a pressing die to form a plate profile. On the plate profile, reference points or collars that protrude up from the plane of the blank are produced, which are used for positioning the plate profile in subsequent production operations. During the forming process, the collars should be formed out of unperforated regions of the blank; the reference points are produced in the form of stamped regions at the edges or in the form of punch-through points or collars within the outline of the plate profile. The hot forming and hardening in the pressing die should generally be advantageous due to the efficient operation achieved by the combination of the forming and the hardening/tempering procedure in a die. But the clamping of the plate profile in the die and the thermal stresses end up producing distortion in the component that cannot be exactly predetermined. This can have a negative impact on the subsequent production operations, which is why the reference points are produced on the plate profile.
DE 197 23 655 A1 has disclosed a method for manufacturing sheet steel products in which a sheet steel product is formed in a pair of cooled dies while it is hot and is hardened into a martensitic structure while it is still in the die so that the dies function as immobilizing means during the hardening. In the regions in which a machining is to take place after the hardening, the steel should be kept in the mild steel range; inserts in the dies are used to prevent a rapid cooling and therefore a martensitic structure in these regions. It should also be possible to achieve the same effect by means of recesses in the dies so that a gap forms between the steel plate and the dies. This method has the disadvantage that due to the considerable amount of distortion that can occur in it, the method in question is unsuitable for press hardening components with a more complex structure.
DE 100 49 660 A1 has disclosed a method for manufacturing locally reinforced formed sheet metal parts; the base plate of the structural part, when flat, is attached to the reinforcing plate in a definite position and this so-called patched composite plate is then formed as a unit. In order to improve the manufacturing method with regard to method creation and the results achieved and in order to relieve stress on the mechanisms executing the method, the patched composite plate is heated to at least approximately 800 to 850° C. before the forming, is rapidly inserted, quickly formed in the hot state, and then cooled in a definite way through contact with the blower-cooled forming die while the formed state is mechanically maintained. Particularly the temperature range of 800 to 500° C., which is decisive in this regard, should be passed through at a definite cooling speed. The step of joining the reinforcing plate to the base plate should be easy to integrate into the forming process; the parts are hard soldered to each other, which can simultaneously produce an effective corrosion protection in the contact zone. This method has the disadvantage that the defined internal cooling renders the dies very complex.
DE 2 003 306 has disclosed a method and device for pressing and hardening a steel component. The object is to press and harden pieces of plate steel in a die, with the intent of avoiding the disadvantages of prior processes, in particular that parts made of sheet steel are manufactured in successive, separate steps for form pressing and hardening. In particular, the intent is to prevent the hardened or quenched articles from deforming in relation to the desired form and thus necessitating additional work steps. To achieve this, a steel piece, after having been heated to a temperature that induces its austenitic state, is placed between a pair of cooperating die elements, whereupon the piece is pressed and at the same time, heat is rapidly dissipated from the piece into the die parts. The die parts are kept at a cool temperature during the entire process so that a quenching action is exerted on the piece while it is subjected to a die pressure.
DE 101 20 063 C2 has disclosed supplying metallic profile motor vehicle components, which are made of a base material supplied in belt form, to a profile rolling unit and rolling them into a rolled profile; after emerging from the profile rolling unit, some regions of the rolled profile are inductively heated to a temperature required for the hardening and then quenched in a cooling unit. After this, the rolled profiles should be cut to the length to form the profile components.
U.S. Pat. No. 6,564,604 B2 has disclosed a process for manufacturing a part with very high mechanical properties in which the part is to be manufactured by stamping a strip out of a rolled steel sheet and in particular, a hot rolled and coated component is coated with a metal or metal alloy intended to protect the surface of the steel and in which the steel sheet is cut to obtain a sheet steel blank; the sheet steel blank is hot formed or cold formed and either cooled and hardened after the hot forming or heated and then cooled after the cold forming. An intermetallic alloy should be deposited onto the surface before or after the forming and should offer a protection against corrosion and decarburization of the steel; this intermetallic mixture can also perform a lubricating function. The excess material is then removed from the blank. The coating here should generally be based on zinc or a zinc-aluminum alloy.
EP 1 013 785 A1 has disclosed a manufacturing process for a component made of rolled steel band, in particular a hot rolled band. The object is to be able to supply rolled steel sheets 0.2 to 2.0 mm thick, which, among other things, are coated after the hot rolling and are subjected to either a hot or cold deformation, followed by a thermal treatment in which the intent is to assure—before, during, and after the hot forming or the thermal treatment—the increase in the temperature without decarburization of the steel and without oxidation of the surface of the above-mentioned sheets. To this end, the sheet is provided with a metal or metal alloy that assures protection of the surface of the sheet, then the sheet is subjected to a temperature increase for the forming, whereupon a forming of the sheet is carried out and the part is then cooled. In particular, the coated sheet should be pressed in the hot state and the part produced by means of deep-drawing should be cooled for hardening purposes, in fact at a speed that is greater than the critical hardening speed. The application cited above also discloses a steel alloy that ought to be suitable, the intent being to austenitize this steel plate at 950° C. before it is deformed and hardened in the die. The coating applied should in particular be comprised of aluminum or an aluminum alloy; this should provide not only an oxidation and decarburization protection, but also a lubricating action. By contrast with the other known methods, this method does in fact make it possible to prevent scale from forming on the sheet metal part after it has been heated to the austenitizing temperature, but a cold forming of the kind discussed in the application mentioned above is essentially impossible with fire-aluminized sheets because the fire-aluminized layer has too low a ductility to permit a greater deformation. Particularly deep-drawing processes for more complex forms cannot be achieved with sheets of this kind when cold. With a coating of this kind, hot forming procedures, i.e. forming and hardening in a single die, are possible, but the component does not have any cathodic protection afterward. In addition, such a component must be machined mechanically or by laser after hardening, thus involving the previously described disadvantage that subsequent machining steps are very expensive due to the hardness of the material. It is also disadvantageous that all of the regions of the formed part that are cut mechanically or by laser no longer have any corrosion protection whatsoever.
From DE 102 54 695 B3, it is known to manufacture a metallic formed component—in particular a body component comprised of a semifinished product that is composed of an unhardened, hot formable steel sheet—by first forming the semifinished product by means of a cold forming process, in particular by means of deep-drawing. Then, the edges of the component blank are cut along an outer contour that approximately corresponds to that of the component to be manufactured. Finally, the cut component blank is heated and press-hardened in a hot forming die. The component produced in this manner has the desired outer contour immediately after the hot forming, thus making it unnecessary to subsequently trim the component edge. This should significantly reduce the cycle times in the manufacture of hardened components of sheet steel. The steel used should be an air-hardened steel that is heated under the protection, as needed, of a protective gas atmosphere in order to avoid scale formation during the heating. Otherwise, a scale layer on the formed component is descaled after the hot forming of the component. The patent application cited above mentions the fact that in the course of the cold forming process, the component blank comes out of the die in a form close to the final contour; the expression “close to the final contour” is intended to signify that the parts of the geometry of the finished component that are accompanied by a macroscopic material flow are completely formed into the component blank after the end of the cold forming process. After the end of the cold forming process, producing the three-dimensional form of the component should require only slight adaptations in shape, which require a minimum of local material flow. This method has the disadvantage that as before, a final forming step of the overall contour still occurs in the hot state and in order to avoid the formation of scale, either the known approach must be taken, which involves annealing in an envelope of protective gas, or the parts must be descaled. Both processes must be followed by a subsequent provision of a corrosion protection coating.
In summary, it is clear that all of the above-mentioned methods share the disadvantage that in order to achieve an optimal cooling action and to avoid distortion, steps are taken to achieve a 100% contact of the formed parts against the dies (a so-called 100% marking image).
Such a marking image requires a long, very labor-intensive breaking-in of the die in which applied ink is used to indicate which regions of the component are not yet resting against the die over their entire surface. Correspondingly, the surface must be continuously corrected. In spite of this fact, all known press hardening processes share the trait that despite careful breaking-in, distortion and cut edge displacement occur frequently and in an unpredictable fashion so that particularly after coming out of the die, components are distorted and the cut edges become displaced. Because of the high degree of hardness, such parts can no longer be remachined and for example straightened. In the known methods, the remachining is limited to the final trimming by laser.
One object of the present invention is to create a method for manufacturing hardened components comprised of sheet steel, which sharply curtails the break-in time of dies, reduces die wear, and supplies distortion-free, reliable components with a high degree of dimensional accuracy and fit, making it possible to omit remachining of the work pieces.
Another object is to create a device for manufacturing hardened components comprised of sheet steel, which has a reduced break-in time, is less susceptible to wear, is quicker to repair, and supplies distortion-free, reliable components with a high degree of dimensional accuracy and fit.